I don't see in the standard any explicit forbidding of negative deltas so it must be correct. My interpretation is that frame 0 is at (all times are offset from beginning) 0ms, frame 1 is at 67.447ms, then frame 2 is at ~45ms, frame 3 is again at 67.447, etc. IOW I believe that the negative offset can reorder frames. Is my understanding correct? What do you think?